Color liquid crystal display apparatus have been expanding the market in recent years as monitors of personal computers including notebook-sized personal computers, and as display sections of liquid crystal television sets including those with an incorporated video player, portable telephones, personal digital assistants and the like. Efforts are being paid to develop high resolution liquid crystal display apparatus having a large size display screen in order to accommodate the increased volume of information to be processed by liquid crystal display apparatus and the diversified demand for liquid crystal display apparatus including the demand for those adapted to a multimedia environment.
A liquid crystal display apparatus basically comprises a backlight unit and a liquid crystal display element part. The backlight unit can be of an under-light type where the primary light source is arranged right under the liquid crystal display element part or of an edge-light type where the primary light source is arranged vis-à-vis a lateral face of the light guide. An edge-light type backlight unit is widely used from the viewpoint of downsizing of the liquid display apparatus.
In the case of relatively small-sized liquid crystal display apparatus that are produced as display sections of portable telephones and other relatively small display sections and adapted to be viewed in a relatively narrow angular range, the backlight units of the edge-light type are designed to limit the diverging angle of the light emitted from the display screen and emit light intensively within a required angular range in order to effectively utilize the quantity of light emitted from the primary light source in the viewpoint of reduction of power consumption.
JP (A)-2001-143515 proposes a light source device adapted to be used with a display apparatus having a limited angular viewing range and designed to emit light intensively within a relatively narrow angular range in order to raise the efficiency of utilization of light from the primary light source and reduce the power consumption by using a prism sheet having elongated prism forming surfaces on the opposite surfaces thereof that is located adjacent to the light emitting surface of the light guide. A plurality of elongated prisms that run in parallel with each other are arranged on each of the opposite surfaces of the prism sheet including a light input surface and a light output surface, the direction of the elongated prisms on the light input surface being made to agree with that of the elongated prisms on the light output surface to show a corresponding positional relationship. With this arrangement, light emitted from the light emitting surface of the light guide with a distribution pattern limited to an appropriate angular range and having a peak in a direction inclined relative to the light emitting surface is made to enter the prism sheet by way of one of the prism faces of the light input surface, and internally reflected by the other prism face. Then, it is subjected to the refraction effect of the elongated prism at the light output surface so that it can be emitted intensively in a required direction with a relatively narrow emission range.
However, while the light source device can intensively emit light with a narrow angular range, it requires the use of a prism sheet that operates as light deflector and on each of the opposite surfaces of which, a plurality of elongated prisms are arranged in such a way that the direction of the elongated prisms on the light input surface is made to agree with that of the elongated prisms on the light output surface to show a corresponding positional relationship. The process of molding such a prism sheet is a complex one.
In JP(A)-10-254371, improvement in luminance in the direction of the normal is intended by defining the angle of inclination α on one of the prism faces of the elongated prisms are set to be 4.7 to 5.7 degrees, and the angle of inclination β on the other prism face to be 34.2 to 35 degrees. However, since the other prism face is defined to be planar, sufficient effect has not been obtained.
JP(A)-9-507584 (corresponding to International Publication WO94/20871) and JP(A)-9-105804 disclose a prism sheet having a convex or concave surface formed as one of the prism faces of the elongated prism. JP(A)-2002-197908 discloses a prism sheet having a plurality of planar surfaces or a single convex curved surface formed as one of the prism faces of the elongated prism. However, none of the above cited patent documents describe an improvement in a light source device using a prism sheet by defining a special relationship between the angle of inclination of one of the prism faces of the elongated prism and the angle of inclination of the other prism face.